The research proposed here is a study of the molecular organization of the gene-sized DNA molecules which make up the macronuclear genome of the protozoan, Oxytricha fallax. These small DNA molecules (500 -25,000 base pairs) arise through a complex process which involves fragmentation and sequence diminution of chromosomal DNA of the germinal micronucleus. The macronucleus carries on all of the gene expression in the vegetative cells, and the macronuclear DNA molecules appear to be organized as self-contained genetic units. The research is divided into several projects which take advantage of the unique genetic organization of the macronuclear DNA, and of the chromosomal origin of these molecules, in order to clarify the molecular organization of genes and the structure of eukaryotic chromosomes. The projects address the following questions: first, what is the size and sequence organization of regions which adjoin the sites of initiation of 5s RNA and rRNA synthesis; second, how does the organization of 5s RNA and rRNA genes compare to the organization of structural genes, especially the genes for actin and tubulin, and third, what regulatory processes are involved with the induction of actin and tubulin synthesis. The analyses will involve gene cloning, restriction nuclease mapping, direct nucleotide sequence determination, nucleic acid hybridization, in vitro mRNA translation and electron microscopy. These studies are designed to provide a better understanding of the size and organization of eukaryotic gene units, and the molecular structures which are involved with the control of gene expression.